Membrane electrode assembly

ABSTRACT

A membrane electrode assembly is provided comprising an ion conducting membrane and one or more electrode layers that comprise nanostructured elements, wherein the nanostructured elements are in incomplete contact with the ion conducting membrane. This invention also provides methods to make the membrane electrode assembly of the invention. The membrane electrode assembly of this invention is suitable for use in electrochemical devices, including proton exchange membrane fuel cells, electrolyzers, chlor-alkali separation membranes, and the like.

This is a division of application Ser. No. 08/948,599 filed Oct. 10,1997 now U.S. Pat. No. 5,879,828.

FIELD OF THE INVENTION

This invention relates to membrane electrode assemblies suitable for usein electrochemical devices, including proton exchange membrane fuelcells, sensors, electrolyzers, chlor-alkali separation membranes, andthe like, and methods for making same.

BACKGROUND OF THE INVENTION

Electrochemical devices, including proton exchange membrane fuel cells,sensors, electrolyzers, chlor-alkali separation membranes, and the like,have been constructed from membrane electrode assemblies (MEAs). SuchMEAs comprise at least one electrode portion, which include a catalyticelectrode material such as Pt in contact with an ion conductivemembrane. Ion conductive membranes (ICMS) are used in electrochemicalcells as solid electrolytes. In a typical electrochemical cell, an ICMis in contact with a cathode and an anode, and transports ions that arcformed at the anode to the cathode, allowing current to flow in anexternal circuit connecting the electrodes. The central component of anelectrochemical cell, such as a fuel cell, sensor, electrolyzer, orelectrochemical reactor, is the 3-layer membrane electrode assembly, orMEA. It consists, in the most general sense, of two catalyzed electrodesbetween which is sandwiched an ion conducting electrolyte, preferably asolid polymer electrolyte for the applications of this invention. This3-layer MEA is in turn sandwiched between two porous, electricallyconducting elements called electrode backing layers (EBLs), to form a5-layer MEA.

MEAs can be used in sensors and hydrogen/oxygen fuel cells. A typical5-layer MEA for use in a hydrogen/oxygen fuel cell might comprise afirst EBL, a first Pt electrode portion, an ICM containing aproton-exchange electrolyte, a second Pt electrode portion, and a secondEBL. Such a five-layer MEA can be used to generate electricity byoxidization of hydrogen gas, as illustrated in the following reactions:

In a typical hydrogen/oxygen fuel cell, the ions to be conducted by themembrane are protons. Importantly, ICMs do not conductelectrons/electricity, since this would render the fuel cell useless,and they must be essentially impermeable to fuel gasses, such ashydrogen and oxygen. Any leakage of the gasses employed in the reactionacross the MEA results in waste of the reactants and inefficiency of thecell. For that reason, the ion exchange membrane must have low or nopermeability to the gasses employed in the reaction.

ICMs also find use in chlor-alkali cells wherein brine mixtures areseparated to form chlorine gas and sodium hydroxide. The membraneselectively transports sodium ions while rejecting chloride ions. ICMsalso can be useful for applications such as diffusion dialysis,electrodialysis, and pervaporization and vapor permeation separations.While most ICMs transport cations or protons, it is known in the artthat membranes can be prepared that are transportive to anions, such asOH⁻.

The ICM typically comprises a polymeric electrolyte material, which mayconstitute its own structural support or may be contained in a porousstructural membrane. Cation- or proton-transporting polymericelectrolyte materials may be salts of polymers containing anionic groupsand nearby fluorocarbon groups.

Fuel cell MEAs have been constructed using catalyst electrodes in theform of applied dispersions of either Pt fines or carbon supported Ptcatalysts. The predominant catalyst form used for polymer electrolytemembranes is Pt or Pt alloys coated onto larger carbon particles by wetchemical methods, such as the reduction of chloroplatinic acid. Thisconventional form of catalyst is dispersed with ionomeric binders,solvents and often polytetrafluoroethylene (PTFE) particles, to form anink, paste or dispersion that is applied to either the membrane, or theelectrode backing material. In addition to mechanical support, it isgenerally believed in the art that carbon support particles providenecessary electrical conductivity within the electrode layer.

In another variation, a catalyst metal salt can be reduced in an organicsolution of a solid polymer electrolyte to form a distribution ofcatalyst metal particles in the electrolyte, without a support particle,which can then be cast onto an electrode backing layer to form thecatalyst electrode.

In a further variation, Pt fines can be mixed directly with a solutionof solvents and polymer electrolyte and coated onto the electrodebacking layer or membrane ICM. However, because of limitations on howsmall the fines can be made, this approach typically results in veryhigh, and therefore expensive, loading of the catalyst.

Various other structures and means have been used to apply or otherwisebring a catalyst in contact with an electrolyte to form electrodes.These MEAs can include: (a) porous metal films or planar distributionsof metal particles or carbon supported catalyst powders deposited on thesurface of the ICM; (b) metal grids or meshes deposited on or imbeddedin the ICM; or (c) catalytically active nanostructured compositeelements embedded in the surface of the ICM.

The prior art teaches that an effective MEA design must maximize contactbetween the catalyst and the ionomer electrolyte in order to obtainhigher efficiency and capacity to handle higher currents. It isreportedly crucial to maximize the three-phase interface between thecatalyst, ionomer and the gaseous reactants which may permeate theionomer. To that end, a primary objective of previous research has beento optimize catalyst utilization by maximizing the surface area ofcatalyst which is in contact with the ion exchange resin or ionomer, inorder to effectively facilitate the exchange of protons between thecatalyst surface site of the redox reactions and the ion conductionmembrane. Catalyst not in direct complete contact with the ionomer hasbeen termed “non-reacting” catalyst.

Nanostructured composite articles have been disclosed. See, for example,U.S. Pat. Nos. 4,812,352, 5,039,561, 5,176,786, 5,336,558, 5,338,430,and 5,238,729. U.S. Pat. No. 5,338,430 discloses that nanostructuredelectrodes embedded in solid polymer electrolyte offer superiorproperties over conventional electrodes employing metal fines or carbonsupported metal catalysts, particularly for sensors, including:protection of the embedded electrode material, more efficient use of theelectrode material, and enhanced catalytic activity.

SUMMARY OF THE INVENTION

Briefly, this invention provides a membrane electrode or membraneelectrode assembly (MEA) comprising an ion conducting membrane (ICM) andone or more electrode layers that comprise nanostructured elements,which further comprise catalytic material, wherein the nanostructuredelements are in incomplete contact with the ICM, that is, whereingreater than 0% and less than 99% of the volume of said elements isembedded in the ICM. This invention also provides methods of making anMEA. The MEA of this invention is suitable for use in electrochemicaldevices, including proton exchange membrane fuel cells, sensors,electrolyzers, chlor-alkali separation membranes, and the like.

In the MEA of the present invention, the catalyst electrodes areincorporated into very thin surface layers on either side of an ionconductive membrane (ICM) and the catalyst electrode particles are inincomplete contact with the ICM. The electrode layers are in the form ofa dense distribution of isolated catalyst particles partiallyencapsulated in the outermost surface of the ICM. One representativemeasure of catalyst utilization is the amount of electrochemical currentin amps generated per milligram of catalyst (Pt) in a hydrogen/oxygencell. It has been discovered that, in spite of the absence of completecontact with the ICM, conductive supports such as carbon particles, oradditional ionomer, catalyst utilization that is several times higherthan previously demonstrated can be achieved where a high density ofcatalyst particles carried on nanostructured supports is localized closeto but partially outside of the surface of the ICM. This resultcontradicts expectations that any catalyst not in contact with anelectrolyte ionomer or ICM is used less efficiently or not used at all.

In another aspect, there are provided methods for preparing a membraneelectrode assembly. One such method comprises the steps of 1)pretreating a membrane comprising a perfluorosulfonic acid polymerelectrolyte by exposure to a non-aqueous solvent, and 2) compressing thepretreated membrane together with electrode particles so as to transferthe electrode particles to a surface of the membrane. A second suchmethod comprises the step of compressing together a membrane whichcomprises an electrolyte and nanostructured elements so as to transferthe elements to a surface of the membrane and thereby to break between5% and 100% of the elements into two or more pieces. A third such methodcomprises the steps of 1) applying nanostructured elements to a surfaceof an electrode backing layer, and 2) joining that surface of theelectrode backing layer to a membrane layer which comprises anelectrolyte.

The MEA of this invention can be made by lamination transfer of thenanostructured elements so as to only partially embed them in thesurfaces of either the ICM or EBL. In one embodiment, partial embeddingis accomplished by carrying out the attachment at low temperatures. Thelow temperature process is preferably accomplished by pretreating theICM by exposure to a solvent, most preferably heptane, just prior toattaching the catalyst coated nanostructured acicular support particles.Static pressing or continuous nip rolling methods can be used. Inanother embodiment, the catalyst support particles, preferably with verylow catalyst loadings of less than 0.1 mg/cm², are generated andtransfered to the ICM surface such that they are broken by compressiveforces into a thin and dense distribution of smaller elements that arepartially embedded in the ICM.

The present invention provides an MEA comprising nanostructured elementsin incomplete contact with an ICM wherein preferably there is a densedistribution of nanoscopic catalyst particles in incomplete contact withan ICM. The nanostructured elements can have one end embedded in an ICMand another end protruding from the ICM. The population ofnanostructured elements may lie partially within and partially outsidean ICM. The methods of making an MEA include a process wherein the ICMis pretreated with a solvent prior to compression with nanostructuredelements to form the electrode layer. A second method includes a processwherein the nanostructured elements are broken by compressive forcesinto a dense distribution of smaller elements during the formation ofthe electrode layer. A third method of making an MEA includes a processwherein the nanostructured elements are applied to an electrode backinglayer which is then joined to an ICM to form an MEA.

In another aspect, the present invention provides an MEA comprisingmicrotextures which increase the effective catalyst surface density ofthe MEA.

In a further aspect, the invention provides a fuel cell assemblycomprising at least one MEA disclosed above.

In yet another aspect, the invention provides an electrochemical devicecomprising at least one MEA disclosed above.

In this application:

“composite membrane” means a membrane composed of more than one materialand including both a porous membrane material and an ion conductingelectrolyte material;

“membrane electrode assembly” means a structure comprising a membranethat includes an electrolyte and at least one but preferably two or moreelectrodes adjoining the membrane;

“microtextures” means surface structures, features or convolutions madeby any process, including impression, molding or etching, whose averagedepth is between 1 and 100 micrometers;

“complete contact” means, with regard to contact between a catalystparticle and an ICM, that the catalyst particle is fully embedded in theICM;

“nanostructured element” means an acicular, discrete, microscopicstructure comprising a catalytic material on at least a portion of itssurface;

“microstructure” means an acicular, discrete, microscopic structure;

“nanoscopic catalyst particle” means a particle of catalyst materialhaving at least one dimension of about 10 nm or less or having acrystallite size of about 10 nm or less, measured as diffraction peakhalf widths in standard 2-theta x-ray diffraction scans;

“acicular” means having a ratio of length to average cross-sectionalwidth of greater than or equal to 3;

“discrete” refers to distinct elements, having a separate identity, butdoes not preclude elements from being in contact with one another; and

“microscopic” means having at least one dimension equal to or smallerthan about a micrometer.

It is an advantage of the present invention to provide an MEA withsuperior catalyst particle density and utilization and superior currentcapacity. In addition, it is an advantage of the present invention toprovide methods of making the MEA of the present invention which arepractical for batchwise or continuous manufacture. Furthermore, it is anadvantage of the present invention to provide a substantiallyself-humidifying cathode, reducing the need to humidify the cathodeoxidant supply.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of cell voltage vs. cathode specific activity for twofuel cells of the present invention and a comparative example.

FIG. 2 is a transmission electron micrograph taken at 300,000×magnification of a cross-section of one surface of an MEA of the presentinvention.

FIG. 3 is a transmission electron micrograph taken at 30,000×magnification of a cross-section of one surface of an MEA of the presentinvention.

FIG. 4 is a graph of Pt crystallite size vs. Pt loading fornanostructured catalyst supports of the present invention.

FIG. 5(a) is a scanning electron micrograph taken at 500× magnificationof a cross section of one surface of an MEA of the present invention.

FIG. 5(b) is a scanning electron micrograph taken at 3000× magnificationof a cross section of one surface of an MEA of the present invention.

FIG. 5(c) is a scanning electron micrograph taken at 30,000×magnification of a cross section of one surface of an MEA of the presentinvention.

FIG. 6 is a graph of current density vs. cell voltage for a fuel cellMEA of the present invention and calculated results for a further fuelcell of the present invention.

FIG. 7(a) is a scanning electron micrograph taken at 15,000×magnification of a cross section of one surface of an MEA of the presentinvention.

FIG. 7(b) is a scanning electron micrograph taken at 50,000×magnification of a cross section of one surface of an MEA of the presentinvention.

FIG. 8(a) is a is a scanning electron micrograph taken at 15,000×magnification of a cross section of one surface of an MEA of the presentinvention.

FIG. 8(b) is a scanning electron micrograph taken at 50,000×magnification of a cross section of one surface of an MEA of the presentinvention.

FIG. 9 is a scanning electron micrograph taken at 30,000× magnificationof a cross section of one surface of an comparative MEA.

FIG. 10 is a graph showing cell voltage vs. current density for threefuel cells of the present invention (A1-3) and three comparative fuelcells (B1-3).

FIG. 11 is a scanning electron micrograph taken at 30,000× magnificationof a cross section of one surface of an MEA of the present invention.

FIG. 12 is a scanning electron micrograph taken at 30,000× magnificationof a plan view (top-down) of one surface of an MEA of the presentinvention.

FIG. 13 is a graph showing cell voltage vs. current density for threefuel cells of the present invention.

FIG. 14 is a graph showing cell voltage vs. current density for fourfuel cells of the present invention.

FIG. 15 is a graph showing cell voltage vs. current density for ninefuel cells of the present invention.

FIG. 16 is a graph showing cell voltage vs. current density for eightfuel cells of the present invention.

FIG. 17 is a graph showing cell voltage vs. current density for six fuelcells of the present invention.

FIG. 18 is a scanning electron micrograph taken at 30,000× magnificationof a cross section of one surface of an MEA of the present invention.

FIG. 19 is a graph showing cell voltage vs. current density for one fuelcell of the present invention and a comparative example.

FIG. 20 is a graph showing CO response over time for CO sensors of thepresent invention.

FIG. 21 is a graph showing CO response vs. relative humidity for COsensors of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention describes a membrane electrode assembly (MEA), inwhich catalyst electrodes are incorporated into very thin surface layerson either side of an ion conductive membrane (ICM) and in which thecatalyst electrode particles are in incomplete contact with the ICM. Theelectrode layers are in the form of a dense distribution of discretecatalyst particles partially encapsulated in the outermost surface ofthe ICM.

The MEA of this invention can be made by lamination transfer of thecatalyst support particles so as to only partially embed them in thesurfaces of either the ICM or EBL. In one embodiment, partial embeddingis accomplished by carrying out the attachment at low temperatures. Thelow temperature process is preferably accomplished by pretreating theICM, which is preferably a perfluorosulfonic acid polymer membrane, byexposure to a solvent, most preferably heptane, just prior to attachingthe catalyst coated nanostructured acicular support particles. Staticpressing or, more preferably, continuous nip rolling methods can beused. In another embodiment, the catalyst coated support particles aregenerated and transfered to the ICM surface such that they are broken bycompressive forces into a thin and dense distribution of smallerelements that are no longer necessarily acicular and are partiallyembedded in the ICM.

The present invention allows much lower catalyst loadings thanpreviously attained, due to improved catalyst utilization and improvedaccess to the catalyst by the gaseous reactants as a result oflocalization of the catalyst in a thinner layer, closer to the surface,on a support that takes less volume. As shown in FIG. 1, the catalystutilization in amps/mg of Pt is 4-5 times higher than previouslyreported with conventional catalysts under the same test conditions.Reducing the amount of catalyst required is very important because ofthe high cost of catalyst materials.

Because the present invention allows the catalyst surface area per unitvolume to be much higher than conventional catalysts, and more of it isaccessible and utilized, the amount of water generated on the cathode ofa ICM fuel cell per unit volume basis of catalyst/membrane interface ishigher and therefore this interface region can be self-humidifying. Thisreduces the need to humidify the cathode oxidant supply and thereforeimproves overall system efficiency.

It has been discovered that it is not necessary to have intimate contactbetween all of the catalyst surface area and polymer electrolyte toobtain superior fuel cell performance. It is shown that a majority ofthe catalyst surface area on the acicular support particles can be tens,hundreds or thousands of Angstroms away from the ICM itself and giveimproved performance.

The process for preparing the MEA involves deposition of catalystmaterial onto oriented acicular support particles previously arrayed onan initial substrate, then transfer of that film of catalyst supportparticles to the surface of the ICM or EBL. The catalyst is applied tothe outer surface of the support particles and the catalyst supportparticles are localized within 2 micrometers of the ICM/EBL interface.FIGS. 2 and 3 show transmission electron micrographs taken atmagnifications of 300,000× and 30,000× of a thin cross-section of onesurface of a catalyst coated ion exchange membrane of one embodiment ofthe current invention. Pt particles are distributed over largernon-conductive, acicular shaped support particles that are randomlyarrayed and partially embedded in the surface of the ICM. The Pt loadingfor the MEA shown in FIGS. 2 and 3 is 0.025 mg/cm². The Pt catalystparticles are seen as black dots, estimated to be less than about 5 nmin size, decorating pieces and fragments of a non-conductive supportmaterial. Some of the support pieces are wholly embedded within themembrane and others are partially embedded. There is no other ionomer orelectrolyte besides the ICM. The support fragments have no spatialcharacteristic in common other than that they are localized within avery thin layer, less than 2 microns thick, at the surface of the solidpolymer electrolyte membrane. For a given catalyst loading (in mg/cm²)the electrochemical activity of the catalyst electrode is directlyrelated to the active surface area of that catalyst. That surface areais in turn determined by the number of catalyst particles and theirsizes, since the smaller the particle the higher the surface area tovolume ratio. For high catalyst activities in fuel cell electrodescatalyst particles with dimensions in the range of 2-10 nm aredesirable.

For the purpose of illustration, if 0.025 mg/cm² of Pt catalyst isdispersed into 2.5 nm diameter particles, distributed into a membranesurface layer 1 micrometer thick, then the number density of particlesin this surface region would be 14×10¹⁷/cm³. This is an order ofmagnitude larger than the number density that would be found for similarsized catalyst particles if they were supported on typical carbonparticles, which occupy a much larger volume, and which are typicallyapplied in layer thickness of at least 10 microns.

The catalyst support of the present invention also shows improved weightper cent loading of catalyst. The acicular support particles of theinstant invention can support much higher weight percentages of catalystwhile the catalyst particle size remains relatively small. Thisdistinguishes commonly used carbon particles. For example, a commoncatalyst currently sold by E-tek, Inc., Natick, Mass., for use in fuelcells is 20 to 40 wt % Pt on Vulcan XC-72 carbon black. Higher weightpercents, beyond 80%, lead to larger catalyst particles and lowerspecific surface area of the catalyst. For example, catalyst particlescomposed of 80% Pt on Vulcan XC-72 carbon black have an average particlesize of 25 nm (see, e.g., the E-tek 1995 Catalog).

In contrast, nanostructured support particles of this invention have amass density of 0.005 mg/cm² and are coated with at least 0.025 mg/cm²of platinum, representing a catalyst wt % of 83.3. (See type B particlesof the examples below.) Higher Pt loading can result in an even greaterwt %. FIG. 2 shows such a loading, where the catalyst particles arestill on the order of 4 nm in size, as shown in FIG. 4. Hence, incontrast to conventional catalyst supports, the instant invention cansupport extremely high wt % loadings of catalyst without loss of thedesirable small sized particles having a high surface area-to-volumeratio. This is due to the acicular shape (high aspect ratio) of theparticles, the large number per unit area contained on the originalsupport substrate, and the tendency of the catalyst to nucleate intosmall particles as it is deposited on the supports by the particulardeposition method used. These are desirable characteristics of thecatalyst support of the instant invention.

The use of nanostructured elements in catalyst electrode layers is onefactor allowing an extremely high weight percent loading of catalyst,while still obtaining small catalyst particles having a high surfacearea-to-volume ratio. This is due to 1) nucleation of the catalyst intosmall distinct particles as it is deposited on the support particles, 2)the density of distinct catalyst particles on the surface of eachelement, 3) the acicular shape of the nanostructured elements, and 4)the large number of elements per unit area.

Nanostructured elements suitable for use in the present invention maycomprise metal-coated whiskers of organic pigment, most preferably C.I.PIGMENT RED 149 (perylene red). The crystalline whiskers havesubstantially uniform but not identical cross-sections, and highlength-to-width ratios. The nanostructured whiskers are conformallycoated with materials suitable for catalysis, and which endow thewhiskers with a fine nanoscopic surface structure capable of acting asmultiple catalytic sites.

Methods for making microstructured layers are known in the art. Forexample, methods for making organic microstructured layers are disclosedin Materials Science and Engineering, A158 (1992), pp. 1-6; J. Vac. Sci.Technol. A, 5 (4), July/August, 1987, pp. 1914-16; J. Vac. Sci. Technol.A, 6, (3), May/August, 1988, pp. 1907-11; Thin Solid Films, 186, 1990,pp. 327-47; J. Mat. Sci., 25, 1990, pp. 5257-68; Rapidly QuenchedMetals, Proc. of the Fifth Int. Conf. on Rapidly Quenched Metals,Wurzburg, Germany (Sep. 3-7, 1984), S. Steeb et al., eds., ElsevierScience Publishers B.V., New York, (1985), pp. 1117-24; Photo. Sci. andEng., 24, (4), July/August, 1980, pp. 211-16; and U.S. Pat. Nos.4,568,598 and 4,340,276, the disclosures of which patents areincorporated herein by reference. Methods for making inorganic-basedmicrostructured layers of whiskers are disclosed, for example, in J.Vac. Sci. Tech. A, 1, (3), July/September, 1983, pp. 1398-1402 and U.S.Pat. No. 3,969,545; U.S. Pat. Nos. 4,252,865, 4,396,643, 4,148,294,4,252,843, 4,155,781, 4,209,008, and 5,138,220, the disclosures of whichpatents are incorporated herein by reference. K. Robbie, L. J.Friedrich, S. K. Dew, J. Smy and M. J. Brett, J.Vac.Sci.Technol.A 13(3),1032 (1995) and K. Robbie, M. J. Brett and A. Lakhtokia,J.Vac.Sci.Technol.A 13(6), 2991 (1995).

Orientation of the microstructures is generally uniform in relation tothe surface of the substrate. The microstructures are usually orientednormal to the original substrate surface, the surface normal directionbeing defined as that direction of the line perpendicular to animaginary plane lying tangent to the local substrate surface at thepoint of contact of the base of the microstructure with the substratesurface. The surface normal direction is seen to follow the contours ofthe surface of the substrate. The major axes of the microstructures canbe parallel or nonparallel to each other.

Alternatively, the microstructures can be nonuniform in shape, size, andorientation. For example, the tops of the microstructures can be bent,curled, or curved, or the microstructures can be bent, curled, or curvedover their entire length.

Preferably, the microstructures are of uniform length and shape, andhave uniform cross-sectional dimensions along their major axes. Thepreferred length of each microstructure is less than about 50micrometers. More preferably, the length of each microstructure is inthe range from about 0.1 to 5 micrometers, most preferably 0.1 to 3micrometers. Within any microstructured layer it is preferable that themicrostructures be of uniform length. Preferably, the averagecross-sectional dimension of each microstructure is less than about 1micrometer, more preferably 0.01 to 0.5 micrometer. Most preferably, theaverage cross-sectional dimension of each microstructure is in the rangefrom 0.03 to 0.3 micrometer.

Preferably, the microstructures have an areal number density in therange from about 10⁷ to about 10¹¹ microstructures per squarecentimeter. More preferably, the microstructures have an areal densityin the range from about 10⁸ to about 10¹⁰ microstructures per squarecentimeter.

Microstructures can have a variety of orientations and straight andcurved shapes, (e.g., whiskers, rods, cones, pyramids, spheres,cylinders, laths, and the like that can be twisted, curved, orstraight), and any one layer can comprise a combination of orientationsand shapes.

The microstructures have an aspect ratio (i.e., a length to diameterratio) preferably in the range from about 3:1 to about 100:1.

Materials useful as a substrate include those which maintain theirintegrity at the temperature and vacuum imposed upon them during thevapor deposition and annealing steps. The substrate can be flexible orrigid, planar or non-planar, convex, concave, textured, or combinationsthereof.

Preferred substrate materials include organic materials and inorganicmaterials (including, for example, glasses, ceramics, metals, andsemiconductors). Preferred inorganic substrate materials are glass ormetal. A preferred organic substrate material is a polyimide. Morepreferably, the substrate is metallized with a 10-70 nm thick layer ofan electrically conductive metal for removal of static charge. The layermay be discontinuous. Preferably the layer is the same metal used tocoat the microstructure whiskers.

Representative organic substrates include those that arc stable at theannealing temperature, for example, polymers such as polyimide film(commercially available, for example, under the trade designation“KAPTON” from DuPont Electronics, Wilmington, Del.), high temperaturestable polyimides, polyesters, polyamids, and polyaramids.

Metals useful as substrates include, for example, aluminum, cobalt,copper, molybdenum, nickel, platinum, tantalum, or combinations thereof.Ceramics useful as a substrate material include, for example, metal ornon-metal oxides such as alumina and silica. A useful inorganic nonmetalis silicon.

The organic material from which the microstructures can be formed may becoated onto the substrate using techniques known in the art for applyinga layer of an organic material onto a substrate, including, for example,vapor phase deposition (e.g., vacuum evaporation, sublimation, andchemical vapor deposition), and solution coating or dispersion coating(e.g., dip coating, spray coating, spin coating, blade or knife coating,bar coating, roll coating, and pour coating (i.e., pouring a liquid ontoa surface and allowing the liquid to flow over the surface)).Preferably, the organic layer is applied by physical vacuum vapordeposition (i.e., sublimation of the organic material under an appliedvacuum).

Useful organic materials for producing microstructures by, for example,coating followed by plasma etching, can include for example, polymersand prepolymers thereof (e.g., thermoplastic polymers such as, forexample, alkyds, melamines, urea formaldehydes, diallyl phthalates,epoxies, phenolics, polyesters, and silicones; thermoset polymers, suchas acrylonitrile-butadiene-styrenes, acetals, acrylics, cellulosics,chlorinated polyethers, ethylene-vinyl acetates, fluorocarbons,ionomers, nylons, parylenes, phenoxies, polyallomers, polyethylenes,polypropylenes, polyamide-imides, polyimides, polycarbonates,polyesters, polyphenylene oxides, polystyrenes, polysulfones, andvinyls); and organometallics (e.g., bis(η⁵-cyclopentadienyl)iron (II),iron pentacarbonyl, ruthenium pentacarbonyl, osmium pentacarbonyl,chromium hexacarbonyl, molybdenum hexacarbonyl, tungsten hexacarbonyl,and tris(triphenylphosphine) rhodium chloride).

Preferably, the chemical composition of the organic-basedmicrostructured layer will be the same as that of the starting organicmaterial. Preferred organic materials useful in preparing themicrostructured layer include, for example, planar molecules comprisingchains or rings over which π-electron density is extensivelydelocalized. These organic materials generally crystallize in aherringbone configuration. Preferred organic materials can be broadlyclassified as polynuclear aromatic hydrocarbons and heterocyclicaromatic compounds.

Polynuclear aromatic hydrocarbons are described in Morrison and Boyd,Organic Chemistry, Third Edition, Allyn and Bacon, Inc. (Boston: 1974),Chapter 30. Heterocyclic aromatic compounds are described in Morrisonand Boyd, supra, Chapter 31.

Preferred polynuclear aromatic hydrocarbons, which are commerciallyavailable, include, for example, naphthalenes, phenanthrenes, perylenes,anthracenes, coronenes, and pyrenes. A preferred polynuclear aromatichydrocarbon is N,N′-di(3,5-xylyl)perylene-3,4,9,10 bis(dicarboximide)(commercially available under the trade designation “C. I. PIGMENT RED149” from American Hoechst Corp. of Somerset, N.J.), herein designated“perylene red.”

Preferred heterocyclic aromatic compounds, which are commerciallyavailable, include, for example, phthalocyanines, porphyrins,carbazoles, purines, and pterins. Representative examples ofheterocyclic aromatic compounds include, for example, metal-freephthalocyanine (e.g., dihydrogen phthalocyanine) and its metal complexes(e.g. copper phthalocyanine).

The organic materials preferably are capable of forming a continuouslayer when deposited onto a substrate. Preferably, the thickness of thiscontinuous layer is in the range from 1 nanometer to about one thousandnanometers.

Orientation of the microstructures can be affected by the substratetemperature, the deposition rate, and angle of incidence duringdeposition of the organic layer. If the temperature of the substrateduring deposition of the organic material is sufficiently high (i.e.,above a critical substrate temperature which has been associated in theart with a value one-third the boiling point, in degrees Kelvin, of theorganic material), the deposited organic material will form randomlyoriented microstructures either as deposited or when subsequentlyannealed. If the temperature of the substrate during deposition isrelatively low (i.e., below the critical substrate temperature), thedeposited organic material tends to form uniformly orientedmicrostructures when annealed. For example, if uniformly orientedmicrostructures comprising perylene red are desired, the temperature ofthe substrate during the deposition of the perylene red is preferablyabout 0 to about 30° C. Certain subsequent conformal coating processes,such as DC magnetron sputtering and cathodic arc vacuum processes, canproduce curvilinear microstructures.

There can be an optimum maximum annealing temperature for different filmthicknesses in order to fully convert the deposited layer tomicrostructures. When fully converted, the major dimension of eachmicrostructure is directly proportional to the thickness of theinitially deposited organic layer. Since the microstructures arediscrete, are separated by distances on the order of theircross-sectional dimensions, and preferably have uniform cross-sectionaldimensions, and all the original organic film material is converted tomicrostructures, conservation of mass implies that the lengths of themicrostructures will be proportional to the thickness of the layerinitially deposited. Due to this relationship of the original organiclayer thickness to the lengths of the microstructures, and theindependence of cross-sectional dimensions from length, the lengths andaspect ratios of the microstructures can be varied independently oftheir cross-sectional dimensions and areal densities. For example, ithas been found that the length of microstructures are approximately10-15 times the thickness of the vapor deposited perylene red layer,when the thickness ranges from about 0.05 to about 0.2 micrometer. Thesurface area of the microstructured layer (i.e., the sum of the surfaceareas of the individual microstructures) is much greater than that ofthe organic layer initially deposited on the substrate. Preferably,thickness of the initially deposited layer is in the range from about0.03 to about 0.5 micrometer.

Each individual microstructure can be monocrystalline orpolycrystalline, rather than amorphous. The microstructured layer canhave highly anisotropic properties due to the crystalline nature anduniform orientation of the microstructures.

If a discontinuous distribution of microstructures is desired, masks maybe used in the organic layer deposition step to selectively coatspecific areas or regions of the substrate. Other techniques known inthe art for selectively depositing an organic layer on specific areas orregions of a substrate may also be useful.

In the annealing step, the substrate having an organic layer coatedthereon is heated in a vacuum for a time and at a temperature sufficientfor the coated organic layer to undergo a physical change, wherein theorganic layer grows to form a microstructured layer comprising a densearray of discrete, oriented monocrystalline or polycrystallinemicrostructures. Uniform orientation of the microstructures is aninherent consequence of the annealing process when the substratetemperature during deposition is sufficiently low. Exposure of thecoated substrate to the atmosphere prior to the annealing step is notobserved to be detrimental to subsequent microstructure formation.

If, for example, the coated organic material is perylene red or copperphthalocyanine, annealing is preferably done in a vacuum (i.e., lessthan about 1×10⁻³ Torr) at a temperature in the range from about 160 toabout 270° C. The annealing time necessary to convert the originalorganic layer to the microstructured layer is dependent on the annealingtemperature. Typically, an annealing time in the range from about 10minutes to about 6 hours is sufficient. Preferably the annealing time isin the range from about 20 minutes to about 4 hours. Further, forperylene red, the optimum annealing temperature to convert all of theoriginal organic layer to a microstructured layer, but not sublime itaway, is observed to vary with the deposited layer thickness. Typically,for original organic layer thicknesses of 0.05 to 0.15 micrometer, thetemperature is in the range of 245 to 270° C.

The time interval between the vapor deposition step and the annealingstep can vary from several minutes to several months, with nosignificant adverse effect, provided the coated composite is stored in acovered container to minimize contamination (e.g., dust). As themicrostructures grow, the organic infrared band intensities change andthe laser specular reflectivity drops, allowing the conversion to becarefully monitored, for example, in situ by surface infraredspectroscopy. After the microstructures have grown to the desireddimensions, the resulting layered structure, which comprises thesubstrate and the microstructures, is allowed to cool before beingbrought to atmospheric pressure.

If a patterned distribution of microstructures is desired,microstructures may be selectively removed from the substrate, forexample, by mechanical means, vacuum process means, chemical means, gaspressure or fluid means, radiation means, and combinations thereof.Useful mechanical means include, for example, scraping microstructuresoff the substrate with a sharp instrument (e.g., with a razor blade),and encapsulating with a polymer followed by delamination. Usefulradiation means include laser or light ablation. Such ablation canresult in a patterned electrode. Useful chemical means include, forexample, acid etching selected areas or regions of the microstructuredlayer. Useful vacuum means include, for example, ion sputtering andreactive ion etching. Useful air pressure means include, for example,blowing the microstructures off the substrate with a gas (e.g., air) orfluid stream. Combinations of the above are also possible, such as useof photoresists and photolithography.

The microstructures can be extensions of the substrate and of the samematerial as the substrate by, e.g., vapor depositing a discontinuousmetal microisland mask onto the surface of a polymer, then plasma orreactive ion etching away the polymer material not masked by the metalmicroislands, to leave polymer substrate posts protruding from thesurface, so long as they are transferable to the ICM.

A preferred method for making an organic-based microstructured layer isdisclosed in U.S. Pat. Nos. 4,812,352 and 5,039,561, the disclosures ofwhich are incorporated herein by reference. As disclosed therein, amethod for making a microstructured layer comprises the steps of

i) depositing or condensing a vapor of an organic material as a thin,continuous or discontinuous layer onto a substrate; and

ii) annealing the deposited organic layer in a vacuum for a time and ata temperature sufficient to induce a physical change in the depositedorganic layer to form a microstructured layer comprising a dense arrayof discrete microstructures but insufficient to cause the organic layerto evaporate or sublimate.

Useful inorganic materials for producing microstructures include, forexample, carbon, diamond-like carbon, ceramics (e.g., metal or non-metaloxides such as alumina, silica, iron oxide, and copper oxide; metal ornon-metal nitrides such as silicon nitride and titanium nitride; andmetal or non-metal carbides such as silicon carbide; metal or non-metalborides such as titanium boride); metal or non-metal sulfides such ascadmium sulfide and zinc sulfide; metal silicides such as magnesiumsilicide, calcium silicide, and iron silicide; metals (e.g., noblemetals such as gold, silver, platinum, osmium, iridium, palladium,ruthenium, rhodium, and combinations thereof; transition metals such asscandium, vanadium, chromium, manganese, cobalt, nickel, copper,zirconium, and combinations thereof; low melting metals such as bismuth,lead, indium, antimony, tin, zinc, and aluminum; refractory metals suchas tungsten, rhenium, tantalum, molybdenum, and combinations thereof);and semiconductor materials (e.g., diamond, germanium, selenium,arsenic, silicon, tellurium, gallium arsenide, gallium antimonide,gallium phosphide, aluminum antimonide, indium antimonide, indium tinoxide, zinc antimonide, indium phosphide, aluminum gallium arsenide,zinc telluride, and combinations thereof).

The microstructures of the preferred embodiment can be made to haverandom orientations by control of the substrate temperature during thedeposition of the initial PR149 layer, as described above. They can alsobe made to have curvilinear shapes by conditions of the conformalcoating process. As discussed in FIG. 6 of L. Aleksandrov, “GROWTH OFCRYSTALLINE SEMICONDUCTOR MATERIALS ON CRYSTAL SURFACES,” Chapter 1,Elsevier, New York, 1984, the energies of the arriving atoms applied bydifferent coating methods, e.g., thermal evaporation deposition, iondeposition, sputtering and implantation, can range over 5 orders ofmagnitude.

It is within the scope of the present invention to modify the methodsfor making a microstructured layer to make a discontinuous distributionof micro structures.

Preferably, the one or more layers of conformal coating material, ifapplied, serve as a functional layer imparting desirable catalyticproperties, as well as electrical conductivity and mechanical properties(e.g., strengthens and/or protects the microstructures comprising themicrostructured layer), and low vapor pressure properties.

The conformal coating material preferably can be an inorganic materialor it can be an organic material including a polymeric material. Usefulinorganic conformal coating materials include, for example, thosedescribed above in the description of the microstructures. Usefulorganic materials include, for example, conductive polymers (e.g.,polyacetylene), polymers derived from poly-p-xylylene, and materialscapable of forming self-assembled layers.

The preferred thickness of the conformal coating is typically in therange from about 0.2 to about 50 nm. The conformal coating may bedeposited onto the microstructured layer using conventional techniques,including, for example, those disclosed in U.S. Pat. Nos. 4,812,352 and5,039,561, the disclosures of which are incorporated herein byreference. Any method that avoids disturbance of the microstructuredlayer by mechanical forces can be used to deposit the conformal coating.Suitable methods include, for example, vapor phase deposition (e.g.,vacuum evaporation, sputter coating, and chemical vapor deposition)solution coating or dispersion coating (e.g., dip coating, spraycoating, spin coating, pour coating (i.e., pouring a liquid over asurface and allowing the liquid to flow over the microstructured layer,followed by solvent removal)), immersion coating (i.e., immersing themicrostructured layer in a solution for a time sufficient to allow thelayer to adsorb molecules from the solution, or colloidals or otherparticles from a dispersion), electroplating and electroless plating.More preferably, the conformal coating is deposited by vapor phasedeposition methods, such as, for example, ion sputter deposition,cathodic arc deposition, vapor condensation, vacuum sublimation,physical vapor transport, chemical vapor transport, and metalorganicchemical vapor deposition. Preferably, the conformal coating material isa catalytic metal or metal alloy.

For the deposition of a patterned conformal coating, the depositiontechniques are modified by means known in the art to produce suchdiscontinuous coatings. Known modifications include, for example, use ofmasks, shutters, directed ion beams, and deposition source beams.

Key aspects of the formed acicular support nanostructures is that theybe easily transferable from the initial substrate into the membrane orEBL surface to form the MEA catalyst electrode layer; they allow morecatalyst particles to be deposited on the surface, preferably at leastan 80 wt % ratio of catalyst particles to the combined weight of supportand catalyst particles; they have sufficient number density and aspectratio to provide a high value of surface area support for the catalyst,at least 10 to 15 times the planar area of the substrate; and the shapeand orientation of the acicular support particles on the initialsubstrate are conducive to uniform coating with catalyst particles.

Key aspects of the catalyst deposition methods are that they result inthe formation of catalyst particle sizes in the several nanometer range,preferably the 2-10 nm range, which uniformly coat at least a portion ofthe outer surface area of the support particles.

In general, nanoscopic catalyst is deposited on the microstructurewhiskers at nucleation sites which grow into catalyst particles. It hasbeen discovered that the size of the resultant catalyst particle is afunction of the initial size of the acicular support and the amount ofcatalyst loading. For the same catalyst loading, in mg/cm², longercatalyst supports will result in smaller catalyst particle sizes,compared to shorter catalyst supports of the same cross-sectionaldimensions. This is illustrated in FIG. 4, which reports the results ofExample 13, below. FIG. 4 shows the size of Pt crystallites deposited onlong (type A, about 1.5 microns long) and short (type B, about 0.5microns long) catalyst support whiskers, labelled “A” and “B”respectively in FIG. 4.

It has been discovered that catalyst utilization can be increased by theuse of smaller catalyst particles in lower loadings of catalyst ontoshorter microstructure supports. Catalyst utilization can be furtherincreased by localizing those catalyst particles in a thinner surfacelayer which is partially non-embedded in the ICM. It has been discoveredthat all of these goals can be achieved simultaneously by making shortermicrostructure supports, coating them with a lower loading of catalyst,and applying the nanostructured elements to an ICM such that they arebroken and fragmented during the application step to form a thinpartially embedded layer. Nanostructured elements useful in this methodare preferably less than 1.0 micrometer in length, more preferably lessthan 0.6 micrometer in length, have aspect ratios of at least 10 andhave a number density of at least 10 per square micrometer. The loadingof catalyst for nanostructured elements useful in this method is lessthan 0.1 mg per square centimeter of the initial nanostructured elementsubstrate area, preferably less than 0.05 mg/cm², and most preferablyless than 0.03 mg/cm².

The ion conductive membrane (ICM) may be composed of any suitable ionexchange electrolyte. The electrolytes are preferably solids or gels.Electrolytes useful in the present invention can include ionicconductive materials, such as polymer electrolytes, and ion-exchangeresins. The electrolytes are preferably proton conducting ionomerssuitable for use in proton exchange membrane fuel cells.

Ionic conductive materials useful in the invention can be complexes ofan alkalai metal or alkalai earth metal salt or a protonic acid with oneor more polar polymers such as a polyether, polyester, or polyimide, orcomplexes of an alkalai metal or alkalai earth metal salt or a protonicacid with a network or crosslinked polymer containing the above polarpolymer as a segment. Useful polyethers include: polyoxyalkylenes, suchas polyethylene glycol, polyethylene glycol monoether, polyethyleneglycol diether, polypropylene glycol, polypropylene glycol monoether,and polypropylene glycol diether; copolymers of these polyethers, suchas poly(oxyethylene-co-oxypropylene) glycol,poly(oxyethylene-co-oxypropylene) glycol monoether, andpoly(oxyethylene-co-oxypropylene) glycol diether; condensation productsof ethylenediamine with the above polyoxyalkylenes; esters, such asphosphoric acid esters, aliphatic carboxylic acid esters or aromaticcarboxylic acid esters of the above polyoxyalkylenes. Copolymers of,e.g., polyethylene glycol with dialky siloxanes, polyethylene glycolwith maleic anhydride, or polyethylene glycol monoethyl ether withmethacrylic acid are known in the art to exhibit sufficient ionicconductivity to be useful in an ICM of the invention.

Useful complex-forming reagents can include alkalai metal salts, alkalaimetal earth salts, and protonic acids and protonic acid salts.Counterions useful in the above salts can be halogen ion, perchloricion, thiocyanate ion, trifluoromethane sulfonic ion, borofluoric ion,and the like. Representative examples of such salts include, but are notlimited to, lithium fluoride, sodium iodide, lithium iodide, lithiumperchlorate, sodium thiocyanate, lithium trifluoromethane sulfonate,lithium borofluoride, lithium hexafluorophosphate, phosphoric acid,sulfuric acid, trifluoromethane sulfonic acid, tetrafluoroethylenesulfonic acid, hexafluorobutane sulfonic acid, and the like.

Ion-exchange resins useful as electrolytes in the present inventioninclude hydrocarbon- and fluorocarbon-type resins. Hydrocarbon-typeion-exchange resins can include phenolic or sulfonic acid-type resins;condensation resins such as phenol-formaldehyde, polystyrene,styrene-divinyl benzene copolymers, styrene-butadiene copolymers,styrene-divinylbenzene-vinylchloride terpolymers, and the like, that areimbued with cation-exchange ability by sulfonation, or are imbued withanion-exchange ability by chloromethylation followed by conversion tothe corresponding quaternary amine.

Fluorocarbon-type ion-exchange resins can include hydrates of atetrafluoroethylene-perfluorosulfonyl ethoxyvinyl ether ortetrafluoroethylene-hydroxylated (perfluoro vinyl ether) copolymers.When oxidation and/or acid resistance is desirable, for instance, at thecathode of a fuel cell, fluorocarbon-type resins having sulfonic,carboxylic and/or phosphoric acid functionality are preferred.Fluorocarbon-type resins typically exhibit excellent resistance tooxidation by halogen, strong acids and bases, and can be preferable forcomposite electrolyte membranes useful in the invention. One family offluorocarbon-type resins having sulfonic acid group functionality is theNafion™ resins (DuPont Chemicals, Wilmington, Del., available fromElectroChem, Inc., Woburn, Mass., and Aldrich Chemical Co., Inc.,Milwaukee, Wis.). Other fluorocarbon-type ion-exchange resins that canbe useful in the invention comprise (co)polymers of olefins containingaryl perfluoroalkyl sulfonylimide cation-exchange groups, having thegeneral formula (1): CH₂₌CH—Ar—SO₂N⁻—SO₂(C_(1+n)F_(3+2n)), wherein n is0-11, preferably 0-3, and most preferably 0, and wherein Ar is anysubstituted or unsubstituted divalent aryl group, preferably monocyclicand most preferably a divalent phenyl group, referred to as phenylherein. Ar may include any substituted or unsubstituted aromaticmoieties, including benzene, naphthalene, anthracene, phenanthrene,indene, fluorene, cyclopetadiene and pyrene, wherein the moieties arepreferably molecular weight 400 or less and more preferably 100 or less.Ar may be substituted with any group as defined herein. One such resinis p-STSI, an ion conductive material derived from free radicalpolymerization of styrenyl trifluoromethyl sulfonylimide (STSI) havingthe formula (II): styrenyl-SO₂N⁻—SO₂CF₃.

ICM's may also be composite membranes, comprising a porous membranematerial combined with any of the above-described electrolytes. Anysuitable porous membrane may be used. Porous membranes useful asreinforcing membranes of the invention can be of any construction havingsufficient porosity to allow at least one liquid solution of anelectrolyte to be infused or imbibed thereinto and having sufficientstrength to withstand operating conditions in an electrochemical cell.Preferably, porous membranes useful in the invention comprise a polymerthat is inert to conditions in the cell, such as a polyolefin, or ahalogenated, preferably fluorinated, poly(vinyl) resin. Expanded PTFEmembranes may be used, such as Poreflon™, produced by Sumitomo ElectricIndustries, Inc., Tokyo, Japan, and Tetratex™ produced by Tetratec,Inc., Feasterville, Pa.

Porous membranes useful in the present invention may comprisemicroporous films prepared by thermally-induced phase separation (TIPS)methods, as described in, e.g., U.S. Pat. Nos. 4,539,256, 4,726,989,4,867,881, 5,120,594 and 5,260,360, the teachings of which areincorporated herein by reference. TIPS films exhibit a multiplicity ofspaced, randomly dispersed, equiaxed, nonuniform shaped particles of athermoplastic polymer, optionally coated with a liquid that isimmiscible with the polymer at the crystallization temperature of thepolymer, preferably in the form of a film, membrane, or sheet material.Micropores defined by the particles preferably are of sufficient size toallow electrolyte to be incorporated therein.

Polymers suitable for preparing films by the TIPS process includethermoplastic polymers, thermosensitive polymers, and mixtures of thesepolymers, so long as the mixed polymers are compatible. Thermosensitivepolymers such as ultrahigh molecular weight polyethylene (UHMWPE) cannotbe melt-processed directly but can be melt-processed in the presence ofa diluent that lowers the viscosity thereof sufficiently for meltprocessing.

Suitable polymers include, for example, crystallizable vinyl polymers,condensation polymers, and oxidation polymers. Representativecrystallizable vinyl polymers include, for example, high- andlow-density polyethylene, polypropylene, polybutadiene, polyacrylatessuch as poly(methyl methacrylate), fluorine-containing polymers such aspoly(vinylidene fluoride), and the like. Useful condensation polymersinclude, for example, polyesters, such as poly(ethylene terephthalate)and poly(butylene terephthalate), polyamides, including many members ofthe Nylon™ family, polycarbonates, and polysulfones. Useful oxidationpolymers include, for example, poly(phenylene oxide) and poly(etherketone). Blends of polymers and copolymers may also be useful in theinvention. Preferred polymers for use as reinforcing membranes of theinvention include crystallizable polymers, such as polyolefins andfluorine-containing polymers, because of their resistance to hydrolysisand oxidation. Preferred polyolefins include high density polyethylene,polypropylene, ethylene-propylene copolymers, and poly(vinylidenefluoride).

Preferred membranes are fluorocarbon-type ion-exchange resins havingsulfonic acid group functionality and equivalent weights of 800-1100,including Nafion™ 117, 115 and 112 membranes. More preferably, Nafion™membranes as received are pretreated by immersing into a) boilingultra-pure H₂O for 1 hour, b) boiling—3% H₂O₂ for one hour, c)boiling—ultra pure H₂O for 1 hour, d) boiling—0.5 M H₂SO₄ for one hour,e) boiling—ultra pure DI H₂O for one hour. The Nafion is then stored inultrapure DI water until use. Prior to forming an MEA, the Nafion isdried by laying it between several layers of clean linen cloth at 30° C.for 10-20 minutes.

Where used, the electrode backing layer (EBL) can be any materialcapable of collecting electrical current from the electrode whileallowing reactant gasses to pass through. The EBLs provide porous accessof gaseous reactants and water vapor to the catalyst and membrane, andalso collect the electronic current generated in the catalyst layer forpowering the external load. The EBL is typically carbon paper or a meshor a porous or permeable web or fabric of a conductive material such ascarbon or a metal. A preferred EBL material is Elat™, obtained fromE-tek, Inc., Natick, Mass. A more preferred material is a porous polymerfilled with conductive particles. The most preferred Elat™ electrodebacking material is about 0.4 mm thick and is designated as carbon only,that is, it contains no metal or catalyst. In one embodiment of thepresent invention, nanostructured elements are attached to an EBL priorto joining the EBL with an ICM to form an MEA.

Nanostructured elements, described herein, are applied directly to thesurface of the ICM or EBL but not embedded in their entirety. Thenanostructured elements may be embedded only so far as necessary tocreate a firm attachment between the particles and the ICM. While asmuch as 99% of the volume of the nanostructured elements may be embeddedwithin the ICM, preferably, no more than 95% of the volume of thenanostructured elements is contained within the ICM, and more preferablyno more than 90%. Most preferably, at least half of the volume of thenanostructured elements is outside of the ICM. In some embodiments, eachnanostructured element may lie partially within and partially outsidethe ICM. In other embodiments, a part of the entire population ofnanostructured elements may lie within the ICM and a part without, withsome particles embedded, others non-embedded, and others partiallyembedded.

The nanostructured elements can be partially embedded in the surface ofthe ICM in a single orientation or in random directions. In the formercase the catalyst coated support particles can be oriented parallel tothe surface of the ICM so that in principle only catalyst on one side ofthe support particles contacts the solid polymer electrolyte, or theycan be oriented more or less perpendicular to the ICM surface and have afraction of their length embedded in the ICM surface, or the catalystcoated acicular-shaped support particles can have any intermediateposition or combination of positions. Furthermore, the nanostructuredelements may be broken or crushed so as to both further reduce theirsize and allow further compaction of the electrode layer.

It has been discovered that the catalyst particles can be applied to theEBL as well as the ICM by the same methods discussed herein to obtain afunctional MEA. As with the ICMs, nanostructured elements are applieddirectly to the surface of the EBL, optionally without additionalionomer or electrolyte. They can be partially embedded in a singleorientation or in random directions, may be broken or crushed in theirfinal state, and may be in incomplete contact with the ICM.

Processes suitable for applying the catalyst particles to the membraneto form the MEA include static pressing with heat and pressure, or forcontinuous roll production, laminating, nip rolling, or calendering,followed by delamination of the initial catalyst support film substratefrom the ICM surface, leaving the catalyst particles embedded.

Nanostructured elements, supported on a substrate, can be transferredand attached to the ICM (or EBL) by applying mechanical pressure andoptionally heat and subsequently removing the original substrate. Anysuitable source of pressure may be employed. A hydraulic press may beemployed. Preferably, pressure may be applied by one or a series of niprollers. This process is also adaptable to a continuous process, usingeither a flat bed press in a repeating operation or rollers in acontinuing operation. Shims, spacers, and other mechanical devicesintermediate between the source of pressure and the particle substratemay be employed for uniform distribution of pressure. The electrodeparticles are preferably supported on a substrate which is applied tothe ICM surface, such that the particles contact the membrane surface.In one embodiment, an ICM may be placed between two sheets ofpolyimide-supported nanostructured films of nanostructured elementswhich are placed against the ICM. Additional layers of uncoatedpolyimide and PTFE sheets are further layered on either side of thesandwich for uniform distribution of pressure, and finally a pair ofstainless steel shims is placed outside of this assembly. The substrateis removed after pressing, leaving the electrode particles attached tothe ICM. Alternately, the electrode particles may be applied directly tothe ICM surface, free of any substrate and without inclusion of anyadditional ionomer, and then pressed into the surface.

The pressure, temperature and duration of pressing may be anycombination sufficient to partially embed the nanostructured elements inthe membrane. The precise conditions used depend in part on the natureof the nanostructured elements used.

In one embodiment, relatively short nanostructured supports coated witha lower loading of catalyst are applied to an ICM under pressure andheat such that they are broken and fragmented during the applicationstep to form a thin partially embedded layer. Preferably, the resultinglayer is less than about 2 micrometers thick; more preferably less than1.0 micrometer, and most preferably less than 0.5 micrometer thick.Nanostructured elements useful in this method are preferably less than1.0 micrometer in length and more preferably less than 0.6 micrometer inlength, and the loading of catalyst for nanostructured elements usefulin this method is less than 0.2 mg per square centimeter of the whiskersubstrate area, preferably less than 0.1 mg/cm², and most preferablyless than 0.05 mg/cm². In this embodiment, a pressure of between 90 and900 MPa is preferably used. Most preferably, a pressure of between 180and 270 MPa is used. Preferably the press temperature is between 80° C.and 300° C., and most preferably between 100° C. and 150° C. Thepressing time is preferably greater than 1 second and most preferablyabout one minute. After loading into the press, the MEA components maybe allowed to equilibrate to the press temperature, at low or nopressure, prior to pressing. Alternately, the MEA components may bepreheated in an oven or other apparatus adapted for the purposePreferably the MEA components are preheated for 1-10 minutes beforepressing. The MEA may be cooled before or after removal from the press.The platens of the press may be water cooled or cooled by any othersuitable means. Preferably, the MEA is cooled for 1-10 minutes whilestill under pressure in the press. The MEA is preferably cooled to underabout 50° C. before removal from the press. A press employing vacuumplatens may optionally be used.

An advantage of using this process for generating the catalyst supportlayer is that the catalyst density will be as uniform as that of theacicular support particles. Obtaining uniform catalyst loadings acrossthe MEA is important in fuel cells to obtain uniform power and minimalhot spots which can cause a cell to fail if a pinhole burns through themembrane. It can be difficult to obtain uniformly dispersed loadings atvery low levels using solution dispersions, inks or pastes of catalystparticles and polymer electrolytes since that requires very dilutesolutions and very thin wet layers, each of which can be difficult tocontrol at high coating speeds because of varying rates of drying. Incontrast, the current process invention of forming a uniform layer ofcatalyst support structures on a temporary substrate, applying thecatalyst to the support structures, then transferring the catalyzedsupport structures into the surface of the ICM or EBL ensures that eventhe lowest catalyst loadings will remain uniformly distributed overarbitrarily large catalyzed areas.

In another embodiment, the MEA can be formed at room temperature andpressures of between 9 and 900 MPa by pretreatment of the ICM with theappropriate solvent. The ICM is preferably a perfluorosulfonic acidpolymer membrane and more preferably a Nafion™ membrane. This allows thewater uptake ability of the ICM to remain high, and hence improves itsconductivity. In contrast, the prior art requires elevated temperaturesto obtain an intimate bond between the catalyst/ionomer layer and theICM. By briefly exposing a perfluorosulfonic acid polymer membranesurface to a solvent, preferably heptane, that catalyst coatednanostructured support particles can be transferred to and partiallyembedded in the ICM from the support substrate, at room temperature.

In this embodiment, a pressure of between 9 and 900 MPa is preferablyused. Most preferably, a pressure of between 45 and 180 MPa is used.Preferably the press temperature is room temperature, i.e. about 25° C.,but may be anywhere between 0 and 50° C. The pressing time is preferablygreater than 1 second and most preferably between 10 seconds and aboutone minute. Since the pressing occurs at room temperature, no preheatingor post-press cool are required.

The ICM is pretreated by brief exposure to the solvent by any means,including immersion, contact with a saturated material, spraying, orcondensation of vapor, but preferably by immersion. Excess solvent maybe shaken off after the pretreatment. Any duration of exposure whichdoes not compromise the ICM may be used, however, a duration of at leastone second is preferred. The solvent used may be chosen from apolarsolvents, heptane, isopropanol, methanol, acetone, IPA, C₈F₁₇SO₃H,octane, ethanol, THF, MEK, DMSO, cyclohexane, or cyclohexanone. Apolarsolvents are preferred. Heptane is most preferred, as it is observed tohave the optimum wetting and drying conditions and to allow completetransfer of the nanostructured catalysts to the ICM surface withoutcausing the ICM to swell or distort. This pretreatment of the ICM may beused with any catalyst particles and is not limited to nanostructuredelements, although they are the preferred catalyst particles.

It has been discovered that the thin electrode layers of the instantinvention can be imparted with microtextures having features sized inthe 1-50 microns range, i.e., smaller than the membrane thickness butlarger than the catalyst support particle, so that the catalyzedmembrane surface is also replicated with these microtextures. FIGS.5(a), 5(b) and 5(c) are scanning electron micrographs of a cross sectionof such an MEA surface where the nanostructured electrode layer conformsto a microtextured shape of 25 micrometer high peaks and valleys, takenat 500×, 5,000× and 30,000×, respectively. The actual electrode layersurface area per unit planar area of MEA (measured normal to thestacking axis of the MEAs) is increased by the geometric surface areafactor of the microtextured substrate. In the example illustrated inFIG. 5, this factor is 1.414, or the square root of two, since each partof the surface is at a 45° angle to the normal stacking axis. However,the resulting increase in MEA thickness is much less than 1.414 and inpractice is negligible. This is so due to interleaving of the ICM/EBLinterface. In addition, the depth of the nanotexture can be maderelatively small compared to the thickness of the ICM, that is, muchless than 0.414 of the thickness of the MEA.

The microtexture can be imparted by any effective method. One preferredmethod is to form the nanostructures on an initial substrate that ismicrotextured. The microtextures are imparted to the MEA during the stepof transferring the nanostructured elements to the ICM, and remain afterthe initial substrate is stripped away. The conditions of nanostructureand MEA formation are the same as described above. Another method is toimpress or mold the microtexture into a formed MEA. It is not necessarythat the microtextures be uniformly geometric. Randomly sized andarrayed features can serve the same purpose.

In a fuel cell, the exchange current density, J₀, is the equilibriumcurrent density equivalent of each half cell reaction pathway under opencircuit conditions. Since it is expressed in terms of amps/unit realarea of catalyst/membrane interface, the exchange current of the fuelcell MEA is also increased by a factor of 1.414. This increase inexchange current per unit planar area of the MEA has the effect ofshifting the polarization curve upward, resulting in an increase involtage at a given current density. Exactly how much depends on theconductivity of the membrane, as indicated in FIG. 6, which illustratesthe effect of increasing the exchange current density relative to theMEA planar area. Trace A of FIG. 6 is a plot of current density vs. cellvoltage for a typical cell. Trace B indicates a calculated increase incurrent density by a factor of 1.414. Since this increase in powerdensity (watts/liter) from the fuel cell was obtained effectivelywithout an increase in geometric size or thickness of the MEA, itrepresents a real and sizeable increase in fuel cell power density of astack of several hundred such MEA's and bipolar plates. In contrast,merely fan-folding the MEA would simply increase the MEA thickness forno net change in power density, i.e. power per unit volume of MEA. Thisincrease in actual catalyst area per unit MEA volume by microtexturingthe catalyst electrode area, can only be achieved when the catalystlayer is sufficiently thin, about an order of magnitude thinner than thesize of the microtexture features, and those microtexture features aresmaller than the thickness of the ion exchange membrane. For example,the thickness of the catalyzed surface region of the ICM in thisinvention can be 2 microns or less. The peak to valley height of themicrotextured features can be 20 microns, and the thickness of the ICMmembrane can be 50 microns or larger. The effect of increased exchangecurrent is larger when the fuel cell operating conditions are limited bythe effect of the cathode overpotential, such as when operating on airat higher cell voltages (such as greater than about 0.7 volts) and lowercell current densities (such as less than about 0.7 amp/cm²). Sincethese are typical design operating goals for fuel cell stacks, this ispotentially a significant advantage of the instant invention.

When the microtextures are imparted by use of a microtextured substratefor the nanostructured whiskers of this invention, two furtheradvantages appear in the process for applying the catalyst and formingthe MEA. A key aspect of the support particles of this invention is thatthey be applied to a substrate from which they can be transferred to themembrane surface. This requirement may result in support particles whichare easily brushed off a flat substrate or damaged by winding up such aflat substrate around a core, such as would be done in a continuous webcoating process. Having the nanostructured catalyst support coated ontoa microtextured substrate can prevent the possibility of damage becausethe vast majority of the much smaller catalyst coated support particleswill reside in the valleys, below the peaks which will protect them fromdamage on roll-up. A second process advantage provided by themicrotextured substrate may be realized in the process of transferringthe catalyzed support particles into the ICM surface. Often heat andpressure may be used, and removing air from the interface at the startof the pressing process can be important, such as by applying a vacuum.When transferring from large pieces of planar substrate carrying thecatalyst support particles, air can be trapped between the ICM and thesupport substrate. Having the microtextured peaks to space the ICM andsubstrate apart during evacuation can allow this air to be moreeffectively removed in the moments just before the press-transfercommences.

This invention is useful in electrochemical devices such as fuel cells,batteries, electrolyzers, electrochemical reactors such as chlor-alkaliseparation membranes, or gas, vapor or liquid sensors, using membraneelectrodes optimized for the immediate purpose.

Objects and advantages of this invention are further illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this invention.

EXAMPLES

There are a number of basic processes and materials common to all theexamples. These include the preparation of the nanostructured catalystsupport, application of the catalyst to the support, determination ofthe catalyst loading, fabrication of the membrane-electrode assembly,the type of fuel cell apparatus and testing station, the fuel cell testparameters, and the kinds of proton exchange membranes or ion conductingmembranes used. These are defined in general as follows:

Nanostructured catalyst support preparation and catalyst deposition. Inthe following examples, the nanostructured catalyst electrodes and theprocess for making them are as described in U.S. Pat. No. 5,338,430,incorporated herein by reference, and other patents referenced therein.The nanostructured catalyst consists of catalyst materials, e.g. Pt orPd, conformally coated onto nanometer sized whisker-like supports. Thewhiskers are produced by vacuum annealing thin films (approximately1000-1500 Angstroms) of an organic pigment material (PR149, AmericanHoechst Co., Somerset, N.J.) previously vacuum coated onto substratessuch as polyimide. The whisker-like supports, with lengths of 1-2micrometers, grow with uniform cross-sectional dimensions of about 30-60nanometers, end-oriented on a substrate to form a dense film of closelyspaced supports (30-40 per square micrometer) which can be transferredinto or onto the surface of a polymer electrolyte to form a catalystelectrode. The nanostructured catalyst electrode has a very high surfacearea which is readily accessible to fuel and oxidant gases.

Measurement of catalyst loading is done both by monitoring the thicknessof the Pt layer deposited during vacuum coating using a quartz crystaloscillator, as is known in the art of vacuum coating, and by a simplegravimetric method. In the latter case, a sample of thepolyimide-supported nanostructured film layer is massed using a digitalbalance accurate to approximately 1 microgram. Then, the nanostructuredlayer is wiped off the polyimide substrate using a paper tissue or linencloth, and the substrate is remassed. Because a preferred property ofthe catalyst support is that it transfer easily and completely to theion conducting membrane, it also can be easily removed by simple wipingwith a cloth. The mass per unit area of the catalyst support particles,without Pt, can also be measured this way.

The ion conducing membranes (ICMs) used were all of the perfluorinatedsulfonic acid type. Nafion™ 117, 115 and 112 membranes were obtainedfrom DuPont, Corp., Wilmington, Del. The Dow chemical membrane (DowChemical Co., Midland, Mich.) tested was a Dow experimental membranedesignated as XUS13204.20, having a dried thickness of approximately 113micrometers.

The process used for transferring the catalyst coated support particlesinto the surface of the membrane was a dry heat and pressure method. Toprepare, e.g. an MEA with 5 cm² of active area, two 5 cm² square piecesof the nanostructured catalyst, coated on metallized polyimidesubstrate, one for the anode, one for the cathode, were placed on eitherside of the center of a 7.6 cm×7.6 cm ICM. The metallized layer on thepolyimide was 10-70 nm thick Pt. One Teflon sheet and one polyimidesheet, each 50 micrometers thick and the same size at least as the ICM,were placed on either side of this stack. Two sheets of 50 micrometerthick polyimide, similarly sized, were placed on the outside of thisstack. This assembly was then placed between two steel plates, 0.25 mmthick, and placed on the vacuum platens of a heated mechanical press. Alow grade vacuum was applied to partially remove air (<2 Torr) frombetween the layers and then the sandwich was pressed at 130° C. at about20,000 Newtons/cm (2.25 tons/cm²) for 1 minute. The press platens werethen cooled to under about 50° C. with the pressure applied beforeopening and removing the sandwich. The original 5 cm² polyimidesubstrates could be easily peeled away from the ICM leaving the catalystembedded in the surface of the ICM.

A similar process was used for applying the catalyst coated supportparticles to the electrode backing layer (EBL). Alternatively, thecatalyst support particles can be transferred to a membrane bycontinuous roll processes such as passing the above sandwich assembliesin continuous or semi-continuous sheet form through the nip of a mill asin calendering or laminating processes. The two mill rolls can beheated, both made of steel, or steel and a softer material such asrubber, have a controlled gap or use controlled line pressure todetermine the gap of the nip.

MEA's prepared as described above were mounted in a 5 cm² fuel cell testcell (Fuel Cell Technologies, Inc., Albuquerque, N. Mex.) using twopieces of 0.38 mm (0.015″) thick ELAT™ electrode backing material(E-tek, Inc., Natick, Mass.). Teflon coated fiberglass gaskets (TheFuron Co., CHR Division, New Haven, Conn.), 250 micrometers thick, with5 cm² square holes cut in the center for the electrode area, were usedto seal the cell. The Elat™ electrode backing material is designated ascarbon only, i.e., it contains no catalyst.

The test cell was attached to a test station also purchased from FuelCell Technologies, Inc. The test parameters for the fuel cellpolarization curves, unless otherwise indicated, were obtained under theconditions of 207 KPa (30 psig) H₂ and 414 KPa (60 psig) oxygen gaugepressures, flowing at about 1 standard liter per minute (SLM).Humidification of the gas streams was provided by passing the gasthrough sparge bottles maintained at approximately 115° C. and 105° C.,respectively, for hydrogen and oxygen. The cell temperature was 80° C.Polarization curves were obtained periodically until they became stable.Pure oxygen was used as the preferred oxidant to show the advantages ofthe catalysts because it allows the polarization curve to be morereflective of the cathode overpotential and hence catalyst activity andless dependent on diffusion limiting processes as occurs with air as theoxidant.

Before use, the Nafion membrane was pretreated by sequentially immersinginto a) boiling water for one hour, b) boiling 3% H₂O₂ for one hour, c)boiling ultra pure H₂O for 1 hour, d) boiling 0.5 M H₂SO₄ for one hour,e) boiling ultra pure DI H₂O for one hour. The Nafion was then stored inultrapure DI water until use. Prior to forming an MEA the Nafion wasdried by laying it between several layers of clean linen cloth at 30° C.for 10-20 minutes. Unless otherwise noted below, the Nafion membrane wasfurther pretreated prior to attachment of electrode material by exposureto reagent grade heptane, usually by briefly dipping the membrane inheptane followed by gently shaking off excess heptane.

Example 1

A 5 cm² membrane electrode assembly (MEA) was prepared as describedabove, using heptane-treated Nafion 117 as the ion-conducting membrane.The pretreated membrane was sandwiched between two pieces ofpolyimide-supported nanostructured catalyst film, prepared as in a),above, having 1500 Å mass equivalent thickness of palladium electronbeam vapor coated on the nanostructured elements. The sandwich assembly,prepared as described above, was pressed at 27 MPa (0.3 tons/cm² ofcatalyst electrode area) for 2 minutes at room temperature. Thepolyimide substrate was peeled away leaving the Pd-coated supportparticles attached to the surface of the membrane. FIG. 7 shows highresolution cross-sectional scanning electron micrographs at 15,000×(FIG. 7(a)) and 50,000× (FIG. 7(b)) magnifications, of the Pd-coatedsupport particles attached to the membrane surface at their tips only.

The micrographs of FIG. 7 show that almost all of the catalyst coatingon the support particles was outside the ICM and that there was no otherionomer or polymer electrolyte in contact with the catalyst. Similarresults could be obtained with platinum coating on the supportparticles.

Example 2

An MEA was prepared as described in Example 1, except that a pressure of160 MPa (1.8 tons/cm²) of electrode area was used for the staticpressing. The polyimide substrate was peeled away leaving the catalystparticles attached to the surface of the membrane as shown in FIG. 8.FIG. 8 shows high resolution cross-sectional scanning electronmicrographs at 15,000× (FIG. 8(a)) and 50,000× (FIG. 8(b))magnifications, of the catalyst particles attached to the ICM surface.

The micrographs of FIG. 8 show that almost all of the catalyst coatingon the support particles remained outside the ICM and that there was noother ionomer or polymer electrolyte in contact with the catalyst. Incontrast to Example 1, however, the catalyst coated support particles ofthis example appeared to be tilted, lying over with more of their tipsembedded in the ICM surface. The overall thickness of the catalyst layerappeared to have been reduced by about 10% compared to the membrane ofExample 1, wherein a lower pressure was used.

Example 3 (Comparative)

An MEA was formed by static hot-pressing catalyst coated supportparticles into the Nafion membrane surface without heptane solventpretreatment, at 130° C. and 160 MPa (1.8 tons/cm² of electrode area)for one minute. FIG. 9 shows an SEM micrograph cross-section, at 30,000×magnification, of the surface region of the MEA. In contrast to Examples1 and 2, no catalyst particles or parts of particles can be seen toextend above the surface of the ICM.

Example 4

Three 10 cm² MEAs were prepared as described in Examples 1 and 2 using,respectively, Nafion 117 (4-1), Nafion 115 (4-2) and Nafion 112 (4-3), acatalyst coating of 1000 Å mass equivalent thickness of platinum, and apressure of 44.5 MPa (0.5 tons/cm²) of electrode area. The degree ofencapsulation was approximately intermediate between that shown in FIGS.7 and 8. Fuel cell polarization curves were obtained as described above,except the air pressure and flow rates for Example 4-3 (Nafion 112) were69 KPa (10 psig) at 1.2 SLM. Polarization curves are shown in FIG. 10Labeled A1, A2 and A3 respectively.

In comparison, three MEAs were prepared by the hot-pressing methoddescribed in Example 3, using Nafion 117 (4-1C), Nafion 115 (4-2C) andNafion 112 (4-3C) as the ICMs, respectively. In other words, no heptanesolvent pretreatment was used in the comparative examples. The catalystsupport particles were fully embedded, similar to that shown in FIG. 9.Fuel cell polarization curves were obtained as described above, exceptthe hydrogen pressure was 34.5 KPa (5 psig) for Examples 4-1C (Nafion117) and 4-2C (Nafion 115), and the air flow rate was 2.5 SLM forExample 4-3C (Nafion 112). Polarization curves are shown in FIG. 10,labeled B1, B2 and B3, respectively. The performance of the fullyembedded MEAs was inferior to that of Examples 4-1, 4-2, and 4-3,prepared according to the process of this invention.

Example 5

In this example, a Nafion membrane was pretreated by dipping in heptanefor one second. An MEA was formed by applying vacuum to the assembly oflayered materials in a Carver press for 2 minutes, then applyingapproximately 89 MPa (1 ton/cm² of electrode area) for 2 minutes at 23°C. FIG. 11 shows a 30,000× SEM micrograph of the catalyst layer astransferred to the Nafion ICM surface. The catalyst coated supportparticles are lying substantially parallel to one another on thesurface.

Example 6

In this example, an MEA with a 50 cm² active electrode area was preparedby nip roll transferring the catalyst layer onto a heptane-pretreatedICM surface at 23° C. A three-layer sandwich was made as follows: A 10.4cm×10.4 cm square of heptane-dipped Nafion 112 was placed between twonanostructured catalyst film layers on 50 micron thick (2 mil) polyimidesubstrates. The catalyst film layer to be the cathode consisted of 0.2mg/cm² of e-beam deposited Pt on nanostructured support particles thatwere, on average, 1.5 micrometer long. The catalyst film layer to be theanode consisted of 0.05 mg/cm² of e-beam deposited Pt on nanostructuredsupport particles that were, on average, 0.5 micrometer long. Thecatalyst coated area on each polyimide substrate was a centered 50 cm²square. The three-layer sandwich was in turn placed between 10additional (5 on each side) 10.4×10.4 cm pieces of 50 micron thickpolyimide. This stack was then passed through the nip of a hand-crankedmill with unheated steel rollers, 7.5 cm in diameter and 15 cm long. Thegap was set at 50 micrometers and the stack passed through the nip atapproximately 3 cm/sec. Mechanical strain-induced separation of thesteel rolls due to play in the hardware was observed. The exact linepressure was not measured. It was observed however, that with the nipgap set initially at 50 microns, an 8 layer stack of 25 micron thicksheets of polyimide would increase the gap when half way through to 175microns, as measured with a feeler gauge.

When the polyimide substrates were removed from the ICM, the catalystwas seen to have transferred cleanly to both sides of the ICM. FIG. 12shows a 30,000× magnification SEM micrograph, looking top-down, at theanode catalyst attached to the surface of an MEA sample prepared asdescribed. (For scale, note that the protruding element portion apparentin FIGS. 2 and 13 is 0.2 micrometer in length.) In the sample, thecatalyzed support particles were lying substantially parallel to oneanother and to the surface, similar to those seen in FIG. 11. Thecathode catalyst was substantially the same in appearance. Trace A ofFIG. 13 shows the H₂/air polarization curve from the 50 cm² MEA of thisexample. A satisfactory current density of 0.6 amps/cm² at 0.7 volts wasobtained on just 34.5 KPa (gauge pressure) (5 psig) air with a total Ptloading of 0.25 mg/cm². The polarization results are especiallysignificant considering, as shown from FIG. 12, that a significantportion of the actual catalyst surface area is not in direct contactwith any ion conducting resin or membrane.

Alternatively, it was shown that the catalyst coated nanostructuredsupport particles can be transferred to the Nafion by nip rolling at 23°C. even without solvent pretreatment of the membrane. Using the handcranked two-roll mill, with the same 50 micrometer (2 mil) gap, athree-layer sandwich was prepared, with the Nafion 117 membrane useddry. The cathode catalyst layer had 0.2 mg/cm² of Pt on 1500 Ånanostructured catalyst supports. The anode catalyst layer had 0.05mg/cm² of Pt on 500 Å nanostructured catalyst supports. For both thecathode and anode, the area of the catalyst coating was a 50 cm² square.Two additional sheets of 50 micron thick polyimide were placed on theoutsides of the catalyst substrates. This stack was passed through themill, at 23° C., at about 3 cm/sec. Transfer of catalyst particles tothe Nafion was observed to be complete. FIG. 13, trace A shows apolarization curve from the 50 cm² MEA of this example. FIG. 13, trace Cshows a polarization curve from a 10 cm² MEA of this example (i.e.,transfer was effected without prior heptane treatment), tested asdescribed above. Trace B shows a comparison polarization curve from asmaller (5 cm²) MEA prepared similarly except it was pretreated with theheptane dip process. The performance of the heptane pretreated sample isbetter in the more critical, higher voltage portion of the curve.

Example 7

Samples were prepared by nip roll transfer of catalyst support particlesto an ICM using a motor driven, controlled-pressure mill with heatedsteel rollers, 15 cm in diameter, moving at 3 ft/min. The nip pressurewas controlled by hydraulic rams on the ends of one roll. Samples of Ptcoated, 1500 Å nanostructured support particles were transferred toheptane-dipped Nafion 117 from 50 micron thick polyimide substrates. Thecatalyst area transferred was a strip approximately 3 to 4 cm wide. Athree-layer stack of catalyst-substrate/membrane/catalyst-substrate wasfed directly, with no other layers, into the nip with the hydraulic rampressure set to either 138 KPa (20 psi) or 552 KPa (80 psi), and rollertemperatures of 38°, 52°, and 66° C., respectively, for each pressure,and at 138 KPa and 79° C. Good transfer was obtained at 138 KPa rampressure at all temperatures. Evidence of Nafion flow was visible on thesamples prepared at 552 KPa such that the catalyst layer was notuniform, indicating that lower ram pressures were preferable.

It was also seen that thinner substrates for the catalyst support film,e.g., 25 micrometer instead of 50 micrometer thick polyimide, affordedmore complete transfer of catalyst to the ICM at low temperatures andpressures, especially for the shorter catalyst support particle films,as described in Example 6. The thinner substrate material is less rigidand more deformable, such that better contact of the catalyst film withthe ICM during the nip-rolling transfer process can be assumed.

Example 8

This example shows that heptane pretreatment can consist of a rapid, onesecond dip or a 5 minute soaking with nearly equivalent transfer ofcatalyst-supported particles.

Two MEAs comprising Nafion 117 ICMs, each approximately 5 cm² in area,were prepared as in Example 6, using a hand-cranked nip roller. One ICMsample was given a 1 second heptane exposure, the other a 5 minute soakin heptane before assembling the sandwich for pressing. Transfer of thecatalysts to both sides of the pretreated membranes appeared verysimilar for both samples; however, the shorter catalyst supportparticles (on the anode side) appeared to transfer slightly lesscompletely on the 5 minute exposure sample than on the single dipexposed sample. However, high resolution SEM micrographs showed thecatalyst support particles were applied to the surfaces of each ICM asseen in FIGS. 11 and 12.

Example 9

MEAs were prepared as described above, using heat and static pressurefor catalyst transfer onto Nafion 117 membranes. The Pt catalyst wascoated onto nanostructured support particles, averaging about 1.5micrometers in length, by electron beam evaporation. Supports of thislength are designated as type A supports. MEAs having Pt loadings of0.215, 0.16, 0.107, and 0.054 mg/cm² (designated 9-1, 9-2, 9-3, and 9-4,respectively) were prepared. Current densities for the MEAs are shown inFIG. 14. The data of FIG. 14 show that, as the Pt loading decreases onthese long supports, fuel cell performance decreases. The currentdensity of over 1.2 amps/cm² at 0.5 volts indicated for Sample 9-1(0.215 mg/cm²), is equal to or exceeds the maximum current density knownin the art for Nafion 117, showing that the power output was membranelimited, even for this low mass loading.

Example 10

In these examples, MEAs were prepared as described above, using heat andstatic pressure for catalyst transfer onto Nafion 115 membranes. The Ptcatalyst was coated onto the nanostructured supports particles byelectron beam evaporation. Two lengths of support particles were used.These are designated as type A, on average 1.5 micrometers in length,and type B, on average 0.5 micrometers in length. The Pt loading ontype. A supports were 0.21, 0.16, 0.11, 0.05, 0.04, and 0.03 mg/cm²,designated 10-1, 10-2, 10-3, 10-4, 10-5, and 10-6, respectively, in FIG.15. For type B supports, the Pt loadings were 0.048 for one membrane(10-7) and 0.025 mg/cm² for two other MEAs (10-8 and 10-9). The data ofFIG. 15 show that, for type A supports, as the Pt loading decreasesbelow approximately 0.05 mg/cm², the fuel cell performance decreases aswell. However, for type B supports, performance remains high even at thelowest catalyst loadings of 0.025 mg/cm².

Example 11

MEAs were prepared as previously described, using heat and staticpressure for catalyst transfer onto Nafion 112 membranes andnanostructured support particles coated with Pt via electron beamevaporation. Two lengths of supports were used, designated as type Asupports, on average 1.5 micrometers in length, and type B supports, onaverage 0.5 micrometers in length. The Pt loading of the type A supportswas 0.21, 0.16, 0.11, 0.05, 0.04, and 0.03 mg/cm², designated 11-1,11-2, 11-3, 11-4, 11-5, and 11-6, respectively, in FIG. 16. For the typeB supports, the Pt loadings were 0.029 (11-7) and 0.0256 mg/cm² (11-8)for two MEAs. The data of FIG. 16 show that, for type A supports, as thePt loading decreases below approximately 0.05 mg/cm², fuel cellperformance decreases, as seen for Nafion 115 membranes of Example 10.However, for type B supports, the performance remains high even at thelowest loading. The current density of 2.25 amps/cm² at 0.5 voltsindicated for a loading of 0.29 mg/cm² (Trace 11-7), is equal to themaximum current density known in the art with Nafion 112 membranes, andto our knowledge, represents the lowest loading ever demonstrated forthis power output. FIGS. 2 and 3 show TEMs of a membrane identical tothose obtained using the type B catalyst supports of this example.

A measure of catalyst utilization can be obtained by normalizing thecurrent density to the mass loading and replotting the polarizationcurve as a function of current per unit mass of Pt, i.e. amps/mg of Pt.This type of plot is a cathode specific activity plot, and polarizationcurves for the type B supports of FIGS. 15 (10-8 and/or 10-9) and 16(11-8) were replotted in this way as traces B and C in FIG. 1. In FIG.1, comparison trace A is shown as obtained under similar cell conditionsand membrane conductivity, but using conventional carbon particlesupported catalysts coated from a dispersion. The cathode specificactivity of the instant invention is seen to be far superior to theprior art, e.g., carbon particle supported catalysts.

Example 12

MEAs were prepared as previously described, using heat and staticpressure for catalyst transfer onto an experimental membrane about 114micrometers thick, made available by Dow and designated XUS13204.20. ThePt catalyst was coated onto nanostructured support particles by electronbeam evaporation. Two lengths of supports were used, designated as typeA supports, on average 1.5 micrometers in length, and type B supports,on average 0.5 micrometers in length. The Pt loading of the type Asupports was 0.21, 0.16, 0.11, and 0.05 mg/cm², designated 12-1, 12-2,12-3, and 12-4, respectively, in FIG. 17. Type B supports were loadedwith 0.044 and 0.029 mg/cm², designated 12-5 and 12-6, respectively. Thedata of FIG. 12 show that type B support particles with the lowest Ptloading gave superior performance at higher current densities. Thecurrent density of >2 amps/cm² at 0.5 volts indicated in FIG. 12 for aloading of 0.029 mg/cm² (12-6), is equal to the maximum current densityknown in the art for this thickness of this Dow membrane, and, to ourknowledge, represents the lowest loading ever demonstrated for thispower output.

Example 13

A series of small MEA's, approximately 1 cm², were prepared explicitlyfor characterization by X-ray diffraction. Pt was electron beam vapordeposited onto four type A (long) and four type B (short) nanostructuredsupports as described previously, at various mass loadings between 0.016mg/cm² and 0.187 mg/cm². Standard 2-theta diffraction scans wereobtained from the MEA samples and from a reference piece of the Nafionmembrane without any catalyst material. Apparent crystallite sizes weredetermined from the Pt(111) diffraction peak half widths aftercorrecting for instrumental broadening and the contribution of theNafion. Peak widths were taken as the full width at half maximum of acalculated peak shape obtained from profile fitting procedures. FIG. 4summarizes the variation of Pt crystallite size with mass loading. Atthese low loadings, high resolution SEM micrographs show that the Ptcrystallites are distinct particles. The data of FIG. 4 show that theshorter, type B, supports give rise to larger crystallite sizesincreasing at a faster rate with increasing Pt loading than the type Asupport. An explanation of the observation is that the amount of surfacearea on the sides of the type A supports is approximately three timesthat of the type B supports, and illustrates how the catalyst particlesize and surface area can be controlled by the controlling the length ofthe acicular support particle.

Example 14

This example demonstrates that very low humidification of the oxidantsupply stream is possible with the instant invention.

An MEA with 0.04 mg/cm² of Pt per electrode was prepared, using heat andstatic pressure for catalyst transfer onto a Nafion 112 membrane.Polarization curves were obtained at 207/414 Kpa (30/60) psig H₂/O₂ andvarying cathode humidification temperatures. The polarization curves forcathode humidity temperatures of, sequentially, 75° C., then 45° C.,then with the sparge bottle by-passed, were identical, producing 2.25Amps/cm² at 0.5 volts. A second, identical MEA was prepared and testedsimilarly, except the oxygen humidification was bypassed from the verystart of testing.

Example 15

This example demonstrates transfer of the acicular shaped catalystcoated support particles uniformly to the surface of a commerciallyavailable electrode backing layer material, ELAT™, identified in (e)above. A 5 cm² square piece of ELAT membrane was placed against aslightly larger piece of 25 micrometer thick polyimide carrying longsupports (1.5 micron) having a loading of 0.2 mg/cm² of Pt. The side ofthe ELAT marked by the manufacturer as normally placed against the ICMin a fuel cell was the side placed against the catalyst film. A piece of25 micron thick polyimide was placed on either side of the pair, and theassembly passed through the nip of the hand-cranked mill described inExample 6. The ELAT membrane was 0.5 mm (0.020 inches) thick. The degreeof catalyst transfer was uniform over the area of the originalsubstrate, but was observed not to be 100% complete. The original blacknanostructured catalyst film coating on the substrate was light gray inappearance after the transfer procedure. The ELAT membrane was next usedas the cathode in an MEA by placing it in the center of a 7.6 cm×7.6 cmsquare of Nafion 117, and placing a 5 cm² piece of Pt catalystnanostructure coated polyimide (0.05 mg/cm²) on the opposite side of theNafion 117 membrane for the anode. The assembly was hot pressed at 130°C. as described in Example 3. The MEA was then tested in a fuel cell asdescribed in (f) above, except it was operated at gauge pressures of207/414 KPa (30/60 psig) of H₂/O₂, respectively. After operating formore than 24 hours, the stabilized polarization curve indicatedsignificant cathode overpotential, but produced 0.1 amps/cm² at 0.25volts and about 0.025 A/cm² at 0.5 volts.

Example 16

This example demonstrates partial embedding of the catalyst coatedsupport particles in the surface of the electrode backing layer (EBL).Nanostructured catalyst coated support particles, 1.5 microns long, wereprepared on 50 micrometer thick polyimide as previously described,having 0.3 mg/cm² of Pt. Samples of approximately 1 cm² area were laidcatalyst side down on a similar sized piece of a carbon loadedpolyolefin EBL prepared as described in applicants' co-pendingapplication Ser. No. 09/208,695, Example 1, filed simultaneouslyherewith, consisting of approximately 95 wt % conductive carbon in aporous membrane of high density polyethylene. Four sheets of 25micrometer thick polyimide were placed on both sides of the sample andthe assembly passed through the hand cranked mill described in Example 6at an initial roller gap of 50 micrometers. The original catalystsupport polyimide substrate was delaminated from the surface of the EBL,leaving the catalyst coated support particles on the surface of the EBLmaterial. FIG. 17 shows an SEM micrograph at 30,000× magnification of across-sectional edge of the sample, clearly showing the acicularcatalyst support particles lying on the surface of the EBL.

In examples 17 and 18 (comparative), the effectiveness of using amicrotextured substrate for the catalyst support and forming thecatalyst surface layer of the ICM with the same pattern is demonstrated.They also demonstrate use of a carbon precoat on the nanostructuresupport particle before Pt deposition to enhance the support particleconductivity, and the use of sputter deposition to apply both the carbonand Pt.

Example 17

A nanostructured catalyst support layer of type A supports was depositedonto a 5 cm² square nickel substrate, 0.25 mm thick, the surface ofwhich was microtextured into a regular array of parallel V-grooves, withpeak-to-peak heights of 20 microns. A mass equivalent thickness of 1500Angstroms of PR149 (per 5 cm² planar area), was vapor deposited onto thesubstrate, then annealed as described above, to produce orientednanostructure supports. A thin carbon precoat was sputter-deposited ontothe oriented supports using a SunSource™ model 7.62 cm (3″) diameter DCmagnetron sputtering source (Material Science, Inc., San Diego, Calif.)operating at 250 watts in 2.4 mTorr of argon. The mass equivalentthickness of the carbon applied was approximately 500 Angstroms. Thisamount would be expected to apply an equivalent coating thickness ofabout 40 Å of carbon around the supports due to the approximately 10 to15 fold increase in geometric surface area provided by the nanostructuresupport particles and a square root of two increase due to the geometricsurface area increase of the microtextured substrate. Similarly, Pt wassputter deposited onto the nanostructure elements with a similar sizedPt target at 300 watts power in 2.3 mTorr of argon, to give aper-unit-planar-area mass loading of 0.165 mg/cm². This 5 cm² coatedmetal substrate was used as the source of cathode catalyst to form theMEA of this example.

For the anode catalyst source, type A supports were deposited and coatedwith a carbon precoat and Pt sputter-coated overlayer, onto a flatpolyimide substrate, as described at the beginning of the examples. Theamount of carbon and Pt loadings (0.165 mg/cm²) were the same as appliedto the cathode substrate of this Example. To form the MEA, the cathodeand anode substrates were placed on either side of a Nafion 115 ICM, 50micron thick polyimide spacer sheets were placed outside these, followedby 125 micron thick metal shims. This sandwich was hot pressed asdescribed above. The microformed metal substrate and the polyimidesubstrate were removed, leaving the respective catalyst layers embeddedin the ICM surface. FIG. 5 shows SEM cross-sections, cut normal to thelength of the V-grooves, at 1500× and 30,000× magnifications of thenanostructure-on-microformed surface layer of the MEA. The MEA wastested on the fuel cell test station at 207 KPa (30 psig) H₂ and air,80° C. cell temperature, 105-115° C. anode humidity temperature and 70°C. cathode humidity. FIG. 19, trace A, shows the polarization curveobtained after over 40 hours of operation. The performance is seen toexceed the comparative example's performance described in Example 18(Trace B, FIG. 19).

Example 18 (Comparative)

An MEA was prepared with catalyst loadings and Nafion membrane identicalto those describe in Example 18, except the cathode catalyst wasprepared on a flat polyimide substrate without any microtexture,identical to the anode catalyst. The fuel cell polarization curve wasobtained under the same testing conditions as Example 18. FIG. 19 showsthe polarization curve obtained after over 40 hours of operation. Theperformance is seen to be poorer than the microformed substrateexample's performance described in Example 18. The high current densityperformance is also seen to be particularly less, indicative of cathodeflooding, suggesting that the microformed cathode catalyst shape hashelped eliminate this effect.

Gas Sensors

U.S. Pat. No. 5,338,430 describes the fabrication and testing of gassensors based on nanostructured electrode membranes in which thenanostructured elements are fully embedded in the surface of the solidpolymer electrolyte. Gas sensors have been prepared by the method ofthis invention, wherein partially encapsulated acicular coated catalystparticles were attached to the surface of a Nafion membrane. Theseexamples 19 and 20 (below) show that the performance of an MEA as anelectrochemical carbon monoxide gas sensor significantly depends on themethod of applying acicular catalyst support particles to the membranesurface. Catalyst coated particles applied by nip-rolling at ambienttemperature, such that the particles were lying substantially parallelto and on top of the surface of the membrane, provided sensors that weresuperior to the case when catalyst coated particles were applied by astatic press method at ambient temperature, such that the particles werepartially encapsulated at their tips but oriented substantially normalto the surface.

Two sets of twelve, two-electrode gas sensors were prepared bytransferring Pt coated nanostructure elements from polyimide substratesto both sides of Nafion 117 ion conducting membranes. In the first set,the transfer method was static pressing at room temperature (coldpress), with no additional solvent, similar to the MEAs made in Example3. In the second set, the transfer method was achieved by nip-rolling atroom temperature as describe in Example 6, above, but with no additionalsolvent treatment of the Nafion. For all sensor samples thenanostructure elements comprised 1.5 to 2 micrometer long acicularsupport particles electron beam coated with 3400 Angstroms of Pt.

Example 19 Cold Pressing

Catalyst support particle transfer was carried out by cold pressing a2.5 cm×5 cm MEA sandwich, as has been described above, using a 15.2 cm(6″) laboratory press (Fred S. Carver Co., Wabash, Ind.) at 138 MPa (10tons/in²) at ambient temperature, for 5 minutes. The Nafion membrane waspretreated and dried as described above. A 0.95 cm dinker die (J. F.Helmold & Bro., Inc., Elk Grove Village, Ill.) was then used to punchout 12 circular sensor elements from the MEA, each 0.95 cm in diameter.The original polyimide substrates were both removed and each sensorelement was installed in a multi-cell two-electrode test chamberdescribed below. SEM micrographs revealed the nanostructure elementswere only partially embedded in the Nafion, and remained substantiallynormal to the membrane surface.

Example 20 Cold Nip-rolling

For the second set of 12 sensor MEAs, the cold rolled MEA from whichthey were die punched was prepared by passing the sandwich through atwo-roll mill equipped with 7.6 cm diameter stainless steel rolls setwith a fixed gap of less than 25 microns, using a ¼ hp motor and aMinarik model SL63 speed controller (Minarik Electric Co., Glendale,Calif.) set at the slowest possible speed. The Nafion membrane waspretreated and dried as described above, then vacuum dried (25 Torr) for45-120 minutes at 23-30° C. The original polyimide substrates were bothremoved, and each sensor element was installed in a multi-celltwo-electrode test chamber described below. SEM micrographs revealed thenanostructure elements were lying on the surface of the Nafion,substantially parallel to the membrane surface.

The test chamber was designed to allow electrical contact to be made toeach electrode of each sensor element, while exposing the workingelectrode but not the counter electrode to the chamber atmosphere. Anelectronic circuit, similar to that described in U.S. Pat. Nos.5,666,949 and 5,659,296, FIG. 12, attached to the multiple sensors heldthe counter electrodes at the same potential as the chamber and biasedthe working electrode while monitoring the cell current. The chamberatmosphere was controlled with respect to flow rates (3 liters/min.),relative humidity, and the concentration of CO (0.1% in N₂, Matheson GasProducts, Secaucus, N.J.) mixed with laboratory air. The CO gasconcentrations were measured at the chamber exit with a Draeger Model190 CO gas detector (National Draeger, Inc., Pittsburgh, Pa.). Allmeasurements were done at ambient temperature, approximately 23° C. Theoutput produced by each sensor was monitored simultaneously with all 12such sensors, as a voltage developed by a differential currentamplifier.

FIG. 20 shows the responses of both sets of sensors when exposed to thesudden introduction of CO into the chamber inlet stream. The chamberrelative humidity (RH) was 50% and the electronic circuit applied a biasof 0.1 volts to the working electrode. For the cold rolled preparedsensors (labeled A in FIG. 20, Example 19), the CO concentration was 82ppm and for the cold pressed (labeled B in FIG. 20, Example 20), the COconcentration was 58 ppm. FIG. 20 shows that the cold rolled preparedsensors exhibited significantly more sensitivity and much lesssensor-to-sensor variability than the cold pressed prepared sensors. Thevariance of the cold rolled sensor responses was 1.8%, versus 43.9% forthe cold pressed sensors. This difference can be attributed to thebetter catalyst/membrane interface characteristics when the catalystsupport particles were partially embedded by lying more parallel to thesurface, than when they were partially embedded by having their tipsembedded in the surface.

A second set of measurements was carried out with both sets of sensors.The sensors were exposed to 100 ppm CO, with the same 0.1 volt bias onthe sensors, and the performance monitored as the relative humidity ofthe inlet stream was varied. FIG. 21 compares the % RH dependence of thecold rolled prepared sensors (A) with the cold pressed prepared sensors(B). FIG. 21 shows that the cold rolled sensors (A) were much morestable with respect to relative humidity.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand principles of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth hereinabove. All publications and patents are hereinincorporated by reference to the same extent as if each individualpublication or patent was specifically and individually indicated to beincorporated by reference.

We claim:
 1. A method of making a membrane electrode assembly comprisinga) a membrane layer which comprises an electrolyte, and b) at least oneelectrode layer, the method comprising the steps of Methods I or III:Method I comprising the steps of: a) pretreating a membrane comprising aperfluorosulfonic acid polymer electrolyte by exposure to a non-aqueoussolvent, and b) compressing together the pretreated membrane andelectrode particles so as to transfer said electrode particles to saidmembrane to provide said membrane assembly; and Method III comprisingthe steps of: a) applying nanostructured elements to a first surface ofan electrode backing layer, and b) joining said first surface of theelectrode backing layer to a membrane layer which comprises anelectrolyte to provide said membrane assembly.
 2. A method of making amembrane electrode assembly comprising a) a membrane layer whichcomprises an electrolyte, and b) at least one electrode layer, themethod comprising the steps of 1) pretreating a membrane comprising aperfluorosulfonic acid polymer electrolyte by exposure to a non-aqueoussolvent, and 2) compressing together the pretreated membrane andelectrode particles so as to transfer said electrode particles to saidmembrane to provide said membrane assembly.
 3. The method according toclaim 2 wherein said compressing occurs at a temperature of no more than50° C.
 4. The method according to claim 3 wherein the solvent is anapolar solvent.
 5. The method according to claim 4 wherein the solventis heptane.
 6. The method according to claim 5 wherein the electrodeparticles are nanostructured elements.
 7. A method of making a membraneelectrode assembly comprising a) a membrane layer which comprises anelectrolyte, b) at least one electrode layer, and c) at least oneelectrode backing layer, the method comprising the steps of 1) applyingnanostructured elements to a first surface of an electrode backinglayer, and 2) joining said first surface of the electrode backing layerto a membrane layer which comprises an electrolyte to provide saidmembrane assembly.